Chimneys which facilitate or create an upward draft or draw produce optimal burning conditions in a fireplace. In addition, an upward draft in chimneys prevents smoke or unburned gas, in wood burning or gas fireplaces, respectively, from entering a room. In a conventional chimney, an upward draft is facilitated or created by the flow of outside air in breezy or windy conditions which draws air up from the fireplace through the chimney flue.
However, many chimneys do not facilitate or create an upward draft or draw for a number of reasons. For example, a chimney which is constructed too low to catch a breeze that is needed to help pull air upward from the fireplace through the flue will not draw properly. On the other hand, chimneys which are too tall do not draw properly since the distance from the fireplace to the chimney flue top may impede the amount of air flow necessary for an upward draft.
Another example of a structural problem which may obstruct the upward draft or draw through a chimney includes an incorrectly sized fireplace or opening between the fireplace and the flue which may prevent or restrict the upward draft. Further, dwellings with insufficiently moving air or no built-in outside air duct in the fireplace will prevent a chimney from drawing properly.
An improperly drawn chimney may also be caused by objects which obstruct the normal flow of outside air, for example a tree limb or a nearby dwelling or hill that is too close to the chimney. Not only do structural problems or objects which obstruct the normal flow of outside air prevent or limit the draw, the problems are exasperated since many times a downdraft of outside air into the chimney is created. Downdrafts contribute to improper burning conditions in the fireplace, as well as blowing smoke or unspent gas into the room.
Patents which disclose fireplace ventilation systems include U.S. Pat. No. 5,609,522 to Szwartz where a combination damper and chimney cap apparatus is installed at the top of the chimney to ventilate a fireplace by providing a draft through the flue. The apparatus includes a platform mounted to the flue which has an aperture for communication with the flue. An element provided in the platform senses temperature and smoke in the flue and at an area external to a fireplace and provides a signal in response to the temperature or smoke. A ventilation fan is connected to the platform to provide a draft sufficient to exhaust the smoke from the fireplace and flue and to cool an electronic motor which drives the apparatus.
U.S. Pat. No. 5,566,667 to Cox discloses a filter and fan assembly for filtering dust and smoke out of hot exhaust gases from a wood burning fireplace. The assembly includes a filter element positioned directly above the flue opening. A fan is located directly above the filter element to ensure that exhaust gases are actively drawn up through the filter.
A chimney stack exhaust treatment unit is disclosed in U.S. Pat. No. 4,236,443 to Schossow. The unit comprises a spherical outer housing which fits over the flue opening and encloses a horizontally mounted fan. The fan is directly over the flue opening so that when it rotates, exhaust gases are drawn up through the chimney and swirled around the inner walls of the outer housing. Exhaust ports near the top of the housing provide an exit for treated gas.
In accordance with the present invention, there is provided a chimney exhaust system which operates on what is referred to as "Bernoulli's Principle." The principle can be simplistically explained in terms of pressure and velocity of air. According to Bernoulli's Principle, energy in an air system, which is a function of pressure and velocity, is constant. Accordingly, an air stream having a higher velocity necessarily has a lower pressure than an air stream in the same system which has a lower velocity. Applying Bernoulli's Principle to the operation of a chimney flue, the velocity of the moving air stream across the opening of the chimney flue creates a lower pressure area at the flue opening. Conversely, since the air in the flue is substantially stagnate, its pressure is necessarily higher than the pressure of the moving air stream across the flue opening. The high pressure air in the flue is drawn to the lower pressure area at the opening of the flue created by the moving air stream. The moving air stream has thus created the draw or upward draft necessary to pull the air up from the fireplace through the chimney flue.